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Polishetty, Ashwin, Goldberg, Moshe and Littlefair, Guy 2010, Wear characteristics of ultrahard cutting tools when machining austempered ductile iron, International journal of
mechanical and mechatronics engineering, vol. 10, no. 1, pp. 1-6.
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International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 10 No: 01
1
Wear Characteristics of Ultra-Hard Cutting
Tools when Machining Austempered Ductile
Iron
Ashwin Polishetty, Dr. Moshe Goldberg and Dr. Guy Littlefair
Abstract—Nodularised Ductile Cast Iron, when subjected to
heat treatment processes – austenitising and austempering
produces Austempered Ductile Iron (ADI). The microstructure
of ADI also known as “ausferrite” consists of ferrite, austenite
and graphite nodules. Machining ADI using conventional
techniques is often a problematic issue due to the microstructural
phase transformation from austenite to martensite during
machining. This paper evaluates the wear characteristics of ultra
hard cutting tools when machining ADI and its effect on
machinability. Machining trials consist of turning ADI
(ASTMGrade3) using two sets of PCBN tools with 90% and 50%
CBN content and two sets of ceramics tools; Aluminium Oxide
Titanium Carbide and Silicon Carbide – whisker reinforced
Ceramic. The cutting parameters chosen are categorized as
roughing and finishing conditions; the roughing condition
comprises of constant cutting speed (425 m/min) and depth of cut
(2mm) combined with variable feed rates of 0.1, 0.2, 0.3 and
0.4mm/rev. The finishing condition comprises of constant cutting
speed (700 m/min) and depth of cut (0.5mm) combined with
variable feed rates of 0.1, 0.2, 0.3 and 0.4mm/rev. The
benchmark condition to evaluate the performance of the cutting
tools was tool wear evaluation, surface texture analysis and
cutting force analysis. The paper analyses thermal softening of
the workpiece by the tool and its effect on the shearing
mechanism under rough and finish machining conditions in term
of lower cutting forces and enhanced surface texture of the
machined part.
industries includes components such as crankshafts,
connecting rods, CV joints, tow hooks and differential spiders
[1]. ADI is ideal for use in high wear applications such as
excavator teeth, mining wear plates and agricultural ground
engagement components due to its high strength and work
hardening capability [2].
A. Background of the material-ADI
The typical microstructure of ADI (ASTM Grade 3) is
shown in Fig.1, where three distinct fractions can be seen
namely: - austenite (white background), ferrite (needle like
structure) and graphite (spheroidal nodules). Table.1 contains
the chemical composition of the ADI (ASTM Grade 3).
Table.2 states the material properties of ADI (ASTM Grade
3).
Ferrite Needles
Graphite Nodule
Index Terms—ADI, Cutting force analysis, Surface texture
analysis, Tool wear analysis
Austenite
I. INTRODUCTION
In recent times, Austempered Ductile Iron (ADI) has been
in increasing demand due to its wide range of balanced and
advantageous material properties, which include high
strength-weight ratio, excellent wear resistance and high yield
strength. Rising demand for materials with high strengthweight ratio makes ADI (Austempered Ductile Iron) an
attractive alternative for materials in different sectors of
engineering as it provides reduced material usage. The
automotive industries use ADI to reduce the weight of
automotive engines and other ancillaries in order to meet the
latest environmental regulations on climate change and
emissions [1].ADI outperforms steel (forged and cast), and
aluminium, in terms of cost per unit yield strength.
As a result, cost reductions can be achieved in high
strength applications. The typical use of ADI in automotive
Fig. 1 Typical microstructure of ADI (ASTM Grade 3)
TABLE.1
TYPICAL CHEMICAL COMPOSITION OF ADI (ASTM GRADE 3)
C
Si
Mn
S
P
Cu
Ni
Mg
3.65
2.8
5
0.1
8
0.00
5
0.02
7
0.9
7
0.0
4
0.04
1
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 10 No: 01
TABLE.2
MATERIAL PROPERTIES OF ADI (ASTM GRADE 3)
Hardness
388 BHN
Ultimate Tensile Strength
1221 N/mm2
Elongation
7.2 %
Fig.2 shows the production of ADI [3]. The two-stage
heat treatment process involves decomposition of austenite to
ferrite and enriched austenite at the first stage. In the second
stage, austempering is conducted in a controlled manner in
order to avoid the decomposition of enriched austenite to
ferrite and carbide. To ensure the ideal combination of
mechanical properties, the heat treatment parameters should
be optimised after the completion of the first reaction (AB)
known as austenitisation at temperatures of 840o C - 950o C
(1550o F – 1750o F) and at the onset of the second reaction
(DE) known as austempering at temperatures of 230o C – 400o
C (450o F – 750o F) [4].
Seah and Sharma, evaluate the machinability of ADI by
integrating an index value as a part of the machinability
assessment [8]. The amounts of unreacted residual austenite
present in the microstructure play an important role in the
machinability of ADI. On machining ADI with high
Unreacted Residual Austenite (URA) content, URA changes
to martensite due to the high residual stress and heat involved.
The surface is work hardened and resists further machining,
eventually leading to tool failure [9]. Yamamoto et al. also
report that strain induced phase transformation leads to poor
machinability of ADI [10]. Another detrimental aspect is that
carbides formed along cell boundaries due to poor casting
quality which are hard and brittle phases have a negative
influence on the machinability of ADI [11]. Katuku et al.
reported wear, cutting force and chip characteristics when dry
turning ADI with PCBN tools under finishing conditions;
depth of cut 0.2mm, feed rate 0.05mm/rev and cutting speeds
ranging from 50 to 800 m/min [12].
II. EXPERIMENTAL PROCEDURE
The aim of the experimental investigation was to establish
the machinability characteristics of ADI using both PCBN and
ceramic cutting tools when machining ADI and also to
evaluate the machining characteristics of ADI (ASTM Grade
3) by analysing tool wear, cutting force and surface texture of
the machined part, as shown in Fig.3.
ADI PCBN
Machining Trials
Tool
A
Fig. 2 A typical austempering cycle
This heat treatment produces a unique microstructure
within the material. The ferrite and carbon stabilized austenite
combine and form alternating layers with a distinctly needle
like appearance known as acicular ausferrite and it is where
ADI obtains its high strength and hardness. Spherical graphite
nodules are found within this matrix of ausferrite and these
promote the good fatigue characteristics. [5].
Research efforts have provided machinists with innovative
techniques to machine less machinable materials. Previous
experimental results and research data on machining studies of
ADI leave numerous gaps regarding the nature of the cutting
mechanism. However, Chang et al, have reported on research
work associated with ADI crankshaft development for
Chrysler automotives [6]. Pashby and Wallbank, reported a
reduction in tool life when machining ADI at elevated cutting
speed for a range of cutting tool materials [7].
2
Cutting Force
Analysis
ADI Ceramic
Machining Trials
Tool
B
Tool
C
Tool Wear
Analysis
Tool
D
Surface texture
Analysis
Comparison analysis between the results for Tool A, B, C and Tool
D
Establishing the machining characteristics of ADI in relation to
selected cutting tools – PCBN and ceramic, subjected to predefined machining criteria
Fig. 3 Flow chart of the experimental design
Experimental design consisted, turning circular ADI
components under extreme and moderate conditions, referred
to henceforth as roughing and finishing conditions.
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 10 No: 01
The roughing conditions comprise of constant cutting
speed (425 m/min) and depth of cut (2mm) combined with
discrete feed rates of 0.1, 0.2, 0.3 and 0.4mm/rev. The
finishing conditions comprise of constant cutting speed (700
m/min) and depth of cut (0.5mm) combined with discrete feed
rates of 0.1, 0.2, 0.3 and 0.4mm/rev.Table.3 tabulates the eight
permutations for the experimental design under roughing and
finishing conditions. The cutting tools in consideration are
two sets of PCBN tools with 90% and 50% Cubic Boron
Nitride (CBN) content and two sets of ceramics tools;
Aluminium Oxide Titanium Carbide and Silicon Carbide –
whisker reinforced Ceramic. Table.4 illustrates the tool
characteristics of PCBN and Ceramic cutting tools. The
cutting tools had their cutting edges rounded and the effect of
the rounded cutting edge geometry on tool wear, surface
texture and cutting force is discussed in the results. Dry
conditions or no coolant was used for the machining trials.
The machine used for the purpose was a Cincinnati Milacron
200/15 turning centre equipped with Kistler platform cutting
force dynamometer (9257B) to measure the three orthogonal
cutting forces.
TABLE.3
MACHINING PARAMETERS FOR ROUGHING AND FINISHING
CONDITIONS
Specification
Type
Tool Name
Tool A - 90%
CBN + 10%
Ceramic binder (
Amborite)
Tool B - 50%
CBN + 50%
Ceramic binder
(DBC50)
Tool C –
Aluminium
oxide (Al2O3) +
Titanium carbide
(TiC)
Tool D – Silicon
carbide single
crystals
(Whisker
reinforced
ceramic)
RNMN 120300T,
6o negative rake
angle
Solid polycrystalline crystal
with high Cubic Boron Nitride
(CBN) content and relatively
coarse grains.
RNMN
120300S0220F, 6o
negative rake angle
Carbide backed polycrystalline
with reduce CBN content and
fine grain structure.
RCGN – 4V T2A
Used for machining of cast iron
and steels which exceeding 32
Rockwell (Rc), at high-elevated
temperature.
RCGN – 4V T1
Exhibits a unique material that
holds advantageous properties
such as high hardness, high
thermal shock resistance and
high melting point
425
425
425
425
Fig. 4 ADI specimen before machining
III. RESULTS AND DISCUSSION
Roughing conditions
1
2
3
4
For the ease and better understanding of the results, the
main task was divided into sub tasks; evaluating the
machining process using the tool wear and cutting force
analysis; and machined component using the surface texture
analysis. Comparison between the results obtained for Tool A,
Tool B, Tool C and Tool D justifies the performance
characteristics of each cutting tool. The machining trials were
performed on a CNC machine programmed with finishing and
roughing operations that compromised one continuous pass of
“metal removal”. One pass of metal removal incorporated
removing material from an ADI casting of 250mm diameter
with a 40mm material thickness dimension, as displayed in
Fig.4. The metal removal process was repeated four times
with various feed rates for either roughing or finishing
machining conditions. This array of machining conditions was
then applied four times, each time using a different cutting
tool, producing in total eight discrete trials for each cutting
tool. After each trial, the cutting inserts were examined under
an optical microscope, in order to evaluate and record the tool
flank wear (VBmax). The cutting tool inserts were replaced each
time in order to provide a fresh cut edge. The surface texture
measurements (Ra) were automatically measured using a well
calibrated electronic instrument (Talysurf VI) in order to
ensure traceability, repeatability and confidence in the results.
Tool Characteristics
TABLE.4
CUTTING TOOLS IN THE EXPERIMENTAL WORK
Cutting
Speed
(m/min)
3
Depth
of Cut
(mm)
2
2
2
2
Finishing conditions
Feed
Rate
(mm/
rev)
0.1
0.2
0.3
0.4
Cutting
Speed
(m/min)
5
6
7
8
700
700
700
700
Depth
of Cut
(mm)
0.5
0.5
0.5
0.5
Feed
Rate
(mm/
rev)
0.1
0.2
0.3
0.4
The obtained results are illustrated graphically and the
results are interpreted according to the pre-defined
machinability assessment criteria. The results from the tool
wear analysis, surface texture analysis and cutting force
analysis on ADI (ASTM Grade 3) subjected to turning
operation using Tool A, Tool B, Tool C and Tool D are all
independently reported.
A. Tool wear analysis
Klocke et al. reported extreme crater wear located very close
to the cutting edge is a characteristics tool wear phenomenon
of ADI [13]. Due to the round nature of the cutting edges,
there was no crater and notch wear but flank wear was present
in differing levels of significance.
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 10 No: 01
Flank wear (Vbmax) for the Tool A, Tool B, Tool C and
Tool D are reported in the form of graphs in Fig.5 and Fig. 6.
4
A. Surface texture analysis
The surface texture assessment was conducted using a
Taylor Hobson, Talysurf VI, surface roughness instrument at a
cut off length of 0.8mm and 90o to the lay. The sampling
length (Meter Cut Off) is sufficiently long to include a reliable
amount of roughness data and yet short enough to exclude
waviness from the measurement. The specimen was mounted
in a flat V-shaped fixture on a granite surface. The arithmetic
roughness parameter (Ra) refers to a numerical value for the
surface. The graphical illustration of the surface texture values
for both roughing and finishing operation is shown in Fig.7
and Fig. 8 respectively.
Fig. 5 Tool wear analysis for roughing operation
Fig. 7 Surface Texture analysis for roughing operation
Fig. 6 Tool wear analysis for finishing operation
The analysis indicates the tool wear developed on the flank
plane of the cutting tool during roughing machining can be
classified as natural low wear. Both Tool D and Tool A
exhibited resilience during the roughing operation. The
analysis of the tool wear developed for finishing machining
condition indicates that Tool D (SiC) and Tool B (50 % CBN
content) were most durable when compared to their
counterparts enduring least damage. In general, the tool wear
appears mostly the consequence of the adhesive/abrasive wear
mechanisms because of the mechanical removal of the surface
layers at low cutting speeds and at high cutting speeds resulted
in diffusion type of wear associated with Fick’s law which
considers issues related to heat transfer and thermo-softening
as time based diffusion factors [14]. The graphs indicate that
the tool wear increases as the feed rate increases. Overall, the
amount of tool wear for the roughing operation was higher
when compared to the finishing operation. Comparing the
roughing and finishing cutting conditions, Tool D proved to
be a versatile tool producing good results.
Fig. 8 Surface Texture analysis for finishing operations
The talysurf instrument automatically measured the Ra
values in this work. The finite radius at the stylus tip fails to
produce a true trace of the surface texture as it physically
unable to penetrate the deepest valleys of the profile resulting
in truncation of the narrow deep valleys. The surface texture
obtained for tool A (90% CBN) was maximum for all
machining conditions. The cutting speed determines the
surface texture condition due to its influence on the
temperature of the tool/workpiece interface. Overall, the
surface texture deteriorated as the feed rate increase – as
would have been expected.
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 10 No: 01
The most optimised surface finish was obtained by
employing cutting speed (700m/min) coupled with feed rate
(0.1mm/rev) and depth of cut (0.5mm).
B. Cutting force analysis
The use of round cutting edges results in the distribution
of the cutting forces as shown in Fig.9.
Axial Force
Tangential
Force
Radial Force
5
force was a dominant force. During machining, the tangential
force was insignificant and played a marginal role and axial
force was less dominant as expected for a turning operation.
Hence, the cutting force analysis to machine ADI using the
Tool A, Tool B, Tool C and Tool D was done with respect to
the radial force. Fig. 10 and Fig. 11 shows the graphical
illustrations of the variation in cutting force for roughing and
finishing operation respectively for the Tool A, Tool B, Tool
C and Tool D. As seen in the graphs, under roughing
conditions, Tool D and Tool A gave the best performance and
under finishing Tool B and Tool D gave the best performance
producing lower cutting forces. The relationship between the
cutting force and the thermo-softening effect of the workpiece
can be seen in the graphs in terms of lower cutting force for
high cutting speeds.
IV. CONCLUSION
The comparative study on wear characteristics of ultra-hard
cutting tools when machining ADI has resulted in the
following inferences.
Tool Motion
Fig.9 Effect of round insert geometry on the distribution of cutting forces
1) Machining ADI requires cutting tool inserts having high
toughness and efficient thermal conductivity. For rough
machining operations, inserts having high CBN content
are required in order to provide fast dissipation of heat.
For finish machining, a relatively low thermal
conductivity insert is required in order to concentrate the
heat in the shear zone leading to softening of the work
piece and reduction of insert wear on the cutting edge.
2) The amount of tool wear generated for the roughing
operation was greater than the finishing operation. The
tool wear graphs indicate that the effect of increase in tool
wear as feed rate increase, which is predominant on
PCBN tools and less effective on ceramic tools. Attrition
wear was dominant at low cutting speeds and diffusion
wear at high cutting speeds.
Fig. 10 Cutting force analysis (radial force) for roughing operation
Fig. 11 Cutting force analysis (radial force) finishing operation
Due to the round cutting edges and the direction of tool
motion, the cutting force measured indicates that the radial
3) Cutting speed plays an important role in determining the
surface texture by controlling the temperature of the
tool/work piece interface. Feed rate demonstrated a
dominant factor on the machined surface texture. The
surface roughness values (Ra) increased at high feed rates
generating greater cusp heights in the profile and viceversa.
4) The benchmark analysis between PCBN and ceramic
tooling indicated that Tool D (SiC) and Tool B (50%
CBN content) are suitable for light cuts, high-speed
machining operations or finishing, whilst Tool D (SiC)
and Tool A (90% CBN content) are suitable for heavy
cuts or rough machining. The cutting tool D (SiC) offers
versatile solution and is suitable for both rough and finish
machining. Machining ADI using Tool C (TiC) has not
produced any advantageous machining results.
5) The cutting speed demonstrated a strong correlation with
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 10 No: 01
the cutting forces. The cutting force was low at high
cutting speeds due to thermo-softening in the shearing
zone. The thermo-softening effect due to the heat
generated around the cutting zone had an impact on finish
machining as low cutting forces were recorded.
6) Overall, ADI is a rapidly emerging material with useful
mechanical properties. The purpose of using ADI in
various design criteria will rely on the research outputs
related to machining the material with ease and
efficiency.
ACKNOWLEDGMENT
The authors acknowledge and like to thank Dr. Graham
Smith, Southampton Institute, Prof. Jhon Berry, Dr. Moshe
Goldberg, ISCAR, Dr. Guy Littlefair and AUT University for
their continual guidance and support.
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